A scanning tunneling microscope (STM) is known in the past as a system effective for observing a crystal structure of a solid surface with a sub-angstrom resolution (.ANG.=10.sup.-10 m).
The conventional STM apparatus employs the system wherein a probe-tip is fixed to the tip of a 3-dimensional fine actuator, and after the distance between the sample and the probe-tip is reduced to some dozens of angstroms to, a few angstroms by use of a coarse actuator (inchworm system) in a Z direction, the position of the probe-tip is subjected to servo control in such a manner as to keep a tunnel current flowing through the probe-tip constant while the inchworm system is fixed and the probe-tip is being scanned in the X and Y directions, in order to display the displacement of the probe-tip at that time. Such a system is disclosed, for example, in U.S. Pat. Specification No. 4,343,993.
On the other hand, a method of obtaining topography during high speed scanning by STM is discussed in Appl. Phys. Lett. 48 (1986), pp. 832-834.
The approaching method of the probe-tip and the sample and the observation area selecting method in accordance with the prior art are described in Appl. Phys Lett. 40 (1982), pp. 178-180.
Incidentally, the principle of STM is described in Scientific American (Japanese version), Oct. 1, 1985, pp. 10-17, and the like.
When high speed scanning is made in accordance with the prior art technique described above, the gap between the probe-tip and the sample is held at an arbitrary distance and after gap control is suspended, the probe-tip is moved on the sample surface and the tunnel current or field emission obtained from the change of the gap is used as the surface information. For this reason, the prior art technique involves the problems in that the structural information of topology cannot be obtained correctly and in that the probe-tip impinges against the sample when any corrugations exist inside the scanning area or the inclination between the scan surface and the sample surface is at least 10 .ANG. or the sample surface grows such as crystal growth especially in the case of large scanning area.
Furthermore, the conventional STM is not free from the problem in that when high speed scanning is made, a fine structure cannot at all be measured because a servo system or a piezo device cannot respond to a high frequency range.
Still another problem of the prior art technique lies in that it does not consider any counter-measure when the change of relative positions between the probe-tip and the sample resulting from the inclination of the sample and its warp exceeds the movable range of the Z-direction fine actuator of the probe-tip or the digitizing error of data. This results in the problems such as the impingement of the sample and the probe-tip and suspension and nullification of measurement.
The prior art technique does not either consider the approach of the probe-tip having a sharp tip to the sample or instability of the tunnel current, and the approach of the probe-tip to the sample while its tip is kept sharp and selection of the observation area have left problems yet to be solved. Disclosure of Invention
It is an object of the present invention to provide a surface metrological apparatus capable of high speed fine characterization even for a sample having a large area.
Unlike the prior art technique which does not subject the probe-tip to servo control in fast scanning but moves it on a predetermined plane to represent a current image, the first action of the present invention subjects the gap between the probe-tip and the sample to servo control even in the case of fast scanning in order to prevent at least the impingement of the probe-tip against the sample. When fast scanning is made, a detection current has an error against a set current value because the frequency response of the control system is not sufficiently high. The present invention makes conversion of current fluctuation to height error for this current and obtains accurate surface information by correcting the structural information or characterization by use of this height error.
The second action of the present invention employs a double servo loop structure consisting of a servo loop which keeps constant the tunnel current by moving a Z-axis fine actuator and a servo loop which controls a Z-axis coarse actuator so as to suppress the low frequency component of the application voltage to the fine actuator. In other words, the invention controls the coarse actuator in such a manner that the Z-axis fine actuator exists always in the movable range during data collection, and thus makes it possible to conduct large area observation.
The third action of the present invention is to make it possible to conduct observation over a wide range without interruption and nullification of data collection by applying, as an offset voltage of a data digitization circuit, the low frequency component of the application voltage to the fine actuator for keeping constant the gap between the probe-tip and the sample and thus shifting the digitization range.
The fourth action of the present invention is directed to having the probe-tip approach the sample without impingement between them and to selecting the observation area by disposing moving means for having the probe-tip closely approach the sample from a further distance or for selecting the observation area together with the fine actuator for keeping constant the gap between the probe-tip and the sample, and by stopping the operation of the moving means immediately after detection of tunnel current more than the set current value and moving it back further an arbitrary distance.